Electronic scales for weighing articles such as letters or parcels are well known. Typically such a scale includes a weighing platform exerting its own (tare) weight on a strain gauge load cell, the output of which is amplified, digitalized, converted to weight units, tare subtracted, and provided to a suitable display, such as a digital display, which indicates the weight of the article, or "live load". The indicated weight can be used to automatically determine a rate (dollar amount) which may ultimately be provided to a postage meter or parcel register for setting a postage or shipping amount for the article. Articles may also be automatically fed by conveying devices to the scale, and from the scale to the meters or register. These external devices, conveyors, meters and registers, all involve mechanical operations that, together with ground vibration from rotating machinery, passing traffic, etc., create a hostile vibratory environment for the scale. These external vibrations, or g-variations are reflected in the output of the load cell (strain gauge and amplifier) as noise, or variations in the indicated weight signal. In weighing operations requiring or benefiting from a high degree of accuracy, noise in the indicated weight signal is unacceptable. For instance, for letters a small increment in indicated weight that is not truly reflective of actual weight can cause more postage than is necessary to be applied to the letter. In high volume mailing applications, an erroneous, albeit small postage increment can become very significant. Other so-called called "hostile" weighing environments would include scales mounted in a vehicle, such as a parcel van with its associated engine vibration; or on shipboard applications where changing tilt angles and engine vibration are present.
Two approaches have been taken to eliminate the effects of ground vibrations on the scale: mechanically isolating the scale from the vibrations, such as by resilient mounting; and electronically compensating for the noise in the output of the load cell. The latter is the subject at hand.
One approach to compensating for noise in the output of a load cell is to filter its output to eliminate noise in the frequency range of the vibrations. However, a tradeoff is ultimately involved in the weighing time since the vibrations are typically low frequency (1-30 hz). U.S. Pat. No. 4,212,361, issued to Stocker in 1980, addresses this issue through the use of a second, reference load cell and at least one frequency dependent network coupling the reference load cell to the weighing load cell and having a transfer characteristic varying as a predetermined function of frequency.
One of the drawbacks of Stocker is that it is limited to noise correction for a particular, reference weight. In postal/shipping operations, a scale may be required to operate with great accuracy over a wide range of weights, such as between 1 ounce and 70 pounds. Other problems attendant Stocker, and the references discussed hereinafter are the stability of the noise correction technique (i.e., nonsusceptability to drift in the reference and/or weighing load cell) and simplicity (typically, simplicity equate with reliability).
U.S. Pat. Nos. 2,767,974 issued to Ballard, 2,767,975 issued to Horst, and 4,396,080 issued to Dee all involve, in one way or another, providing a reference load cell, and subtracting its output from the output of the weighing load cell to "cancel out" the noise in the weighing load cell output. Dee includes a filling-up-type weighing operation, wherein only a discrete, predetermined article weight is pertinent. Ballard involves adjusting the output of the weighing load cell to match that of the reference load cell as loaded with a predetermined weight--the extent of adjustment required to match the two outputs being a measure of the unknown weight on the weighing load cell. Horst involves simple subtraction which, as stated hereinbefore, is pertinent, or accurate, for eliminating noise for only a particular weight. EPC Application No. 84302548.7 is also cited as being limited by the "subtraction" technique.
U.S. Pat. No. 3,322,222 issued to Bauer discloses a reference load sensor, the output of which is divided into the output of the main load sensor, in the context of an electromagnetic balance, to eliminate weighing errors attributable to gross gravitational variations, such as due to changes in altitude, field deterioration in the magnets, vibrations, and scale inclination. U.S. Pat. No. 4,258,811 issued to Franzon also discloses dividing the output of a reference cell into the weighing cell.
U.S. Pat. No. 4,624,331 issued to Naito discloses obtaining a noise level from the reference cell, by removing the DC (steady state) component thereof, adjusting the gain applied to the reference cell noise level, and adding the gain adjusted noise output of the reference cell in opposite phase relation (i.e. subtracting to the output of the unloaded weighing cell. The signal resulting from this addition is multiplied by the noise level of the reference cell, and the result is gain adjusted to match the noise level (DC component removed) of the weighing cell. A multiple stage comparator effects the latter gain adjustment in dependence upon the weight of the article being weighed to obtain a corrected noise signal which is ultimately subtracted from the noisy output of the weighing cell. Naito addresses the problem of providing effective noise cancellation over a range of live loads in a very complex manner, particularly the gain adjustments involved, and is limited in that errors, such as drift, in the reference cell channel will cause proportional errors in the correction term applied to the weighing cell output.
Thus, what is needed is a technique for eliminating noise from the output of a weighing cell that is simple, relatively unaffected by any drift, and that exhibits inherent veracity over a wide range of live loads applied to the weighing cell. It is an object of the present invention to achieve these characteristics.